剑指Offer之Java算法习题精讲二叉搜索树与数组查找

题目一

 解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int ans;
    int pre;
    public int minDiffInBST(TreeNode root) {
        ans = Integer.MAX_VALUE;
        pre = -1;
        method(root);
        return ans;
    }
    public void method(TreeNode root){
        if(root==null) return;
        method(root.left);
        if(pre==-1){
            pre = root.val;
        }else{
            ans = Math.min(ans,root.val-pre);
            pre = root.val;
        }
        method(root.right); 
    }
}

题目二

 解法

class Solution {
    public int dominantIndex(int[] nums) {
        int f = Integer.MIN_VALUE;
        int fi = 0;
        int s = Integer.MIN_VALUE;
        int si = 0;
        for(int i = 0; i<nums.length;i++){
            if(nums[i]>f){
                s = f;
                f = nums[i];
                fi = i;
            }else if(nums[i]>s){
                s = nums[i];
            }
        }
        if(nums.length==1) return 0;
        if(2*s<=f) return fi;
        return -1; 
    }
}

题目三

解法

class Solution {
    public int repeatedNTimes(int[] nums) {
        int n = nums.length/2;
        HashMap<Integer,Integer> map =new HashMap<Integer,Integer>();
        for(int key : nums){
            if(map.containsKey(key)){
                map.put(key,map.get(key)+1);
                if(map.get(key)==n){
                    return key;
                }
            }else{
                map.put(key,1);
            }
        }
        return 0;
    }
}

 题目四

 解法

class Solution {
    public boolean uniqueOccurrences(int[] arr) {
        int[] nums = new int[2000];
        for(int i =0;i<arr.length;i++){
            nums[arr[i]+1000]+=1;
        }
        HashSet<Integer> set =new HashSet<Integer>();
        for(int i =0;i<nums.length;i++){
            if(nums[i]==0) continue;
            if(!set.add(nums[i])){
                return false;
            }else{
                set.add(nums[i]);
            }
        }
        return true;
    }
}

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